Yes it is used by some people as a preventive fungicide and the instructions that come along with the bottle even suggest once a month preventive applications. It may not cure every disease or fungus that strike your orchids, but it can be a big help with some pathogens and it is excellant in stopping the transmission of these organism's from one plant to the next. AL

Can you use Physan 20 as a preventive say for example if your not sure the plant has the fungus? Also, if you have applied it on a affected plant. How long afterward do you have to quarantine the plant?

Yes. In fact it was Alan Moon, then the Curator ( head grower) at the great Ericy Yoing Orchid Foundation who told me that they sprayed all of their orchids, once a month, with a Physan spray, as a prophylactic measure, and that he thought their foliage was cleaner as a result. Goodness knows how much they used, at that time their growing collection was housed in five greenhouses which I think measured each 120 feet long x 30 feet : i have not been since they rebuilt....don't know the present staff, although I belive that anyone who can show they are a member of a genuine orchid society is welcomed and can see behind the display house, which is something else.
Of course it is Selgiene I use now just because Physan is not on sale in UK or the EU for what I call bureaucratic nonsensical reasons.

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And quarantine ? Thats another question. As long as the plant could infect others I guess !

You are lucky to have Physan available. Physan 20 is I think the version designed for higher temperature areas (the tropics) and ordinary Physan for temperate zone areas. It is great for plant hygiene, which means cleaner stronger healthier plants.
Unfortunately it is no longer sold here in UK ( it is not "licensed" and doing all the work to get a licence would cost , so I am told US$400,000...due to bureacracy - which gets me steamed up....) and the cost of a trip to New York, for however many gallons I could get in my luggage to bring home is just not on - but I did consider it !
I used it from about 1990 onwards, until the supply stopped last year. I always added some to the water used for my orchids or for spraying ,at the rate of anything from 1:5000 ( in water/nutrient mix) down to about1:1000 which is approx 1 teaspoon in 1 gallon for spraying all the foliage once a month . It is a fungicide, insecticide, miticide, etc - or in other words a disinfectant which most pests will dislike, so that they don't hang around and don't spread. It is also a very good wetting agent , which means that the water really spreads around and there are no dry corners.
This practice - the monthly spray - started when I visited and was told that this is what they do at the Eric Young Foundation which has one of the best grown orchid collections in the world. What's good enough for them, will do for me.
Nowadays I use another similar product which is licensed in UK - one actually intended for sterilising surfaces used for food preparation - almost identical chemical formulation, and a lot cheaper per gallon ( Selgiene) but I have to use it a bit stronger down to 1:500.
Go for Physan !

I know this is an old thread but I wanted you to know I've been using my Physan (substitute) in all my plant water at about 1/5 teaspoon per gallon for some plants that had issues. One that I knocked out of the pot and damaged a bit and one that I was scared had pseudobulb rot coming into the back bulb. Dowiana influenced plant that I had fed a few times in winter. Minor surgery required for both patients as well. A month and a half on, there are no ill effects that I can see and both plants are growing like gangbusters with lots of long green root tips. Do you still use it or the comparable product? Here you can buy it as an algaecide at a big box home improvement store for cheaper and at the same concentration. I've also heard of it being used a few drops per gallon in grow room humidifiers.

As you know my collection is not large. So far, I've only had an issue with one plant that I'm aware of. It was Epicattleya Tropical Jewel 'Hihimanu'. The plant was half Epi. stamfordianum and the leaves were a bit more tender than most of my other plants. I'm not absolutely sure it was the Physan that caused the problem though. Could have also been all the misting I was doing at that time. (I don't mist foliage any more, it was a brief foray) The plant did have problems from the beginning, hence the Physan. If I ever decided to try these newfangled biological products, I'd give the Physan use up, at least in any plant pots. Guessing it's not a good combo.

I've noticed that the rain water collected in my barrel is sometimes green. Not sure if this green -- which I assume is due to algae -- affects the plants in anyway when I water them. Should/can I add some physan just to keep the algae down?

I've noticed that the rain water collected in my barrel is sometimes green. Not sure if this green -- which I assume is due to algae -- affects the plants in anyway when I water them. Should/can I add some physan just to keep the algae down?

Algae, per se, does not affect your plants. One might argue that dying organic matter within the potting medium is bad, but frankly, I doubt that there's enough to be of any consequence. I used to add Physan to my RO tank for that purpose, but some plants (notably dendrobiums and thin-leaved plants like coelogyne, for example) don't like it.

If you can fins a way to block the light getting to the barrel contents, that'll take care of it.

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